1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, an image forming apparatus which heat-fixes a toner image formed on a sheet.
2. Description of the Related Art
In the past, an electrophotographic image forming apparatus transfers a toner image to a sheet, the toner image being formed on an image bearing member such as a photosensitive drum or an intermediate transfer belt, melts the toner image by heating and pressurizing the sheet, and fixes the toner image on the sheet. Also, in the case of forming a color image, four toners of yellow, cyan, magenta, and black are transferred to a sheet in a superimposing manner and melts the superimposed transferred toner image by heating and pressurizing the sheet to fix the toner image on the sheet.
At this time, a color image used in a photograph or the like requires higher color developing property and gloss property. Therefore, color toners of yellow, cyan, and magenta are required to be more sharp-melted at a low temperature side. However, a toner having a high sharp-melt property tends to be vulnerable to hot offset. Hence, in a case where a color image used in a photograph or the like is formed on a sheet, a wax-containing toner is used for improving toner parting properties and preventing hot offset.
Generally, it is known that a gloss of an image varies depending on cooling states of a toner and a wax after fixing a wax-containing toner. Therefore, if a member such as a conveying roller is brought into contact with a toner image on a sheet where a thermally fixed wax is not solidified, a difference in the cooling state of the wax occurs between a contact portion and a non-contact portion, causing gloss unevenness (hereinafter, referred to as “roller mark”).
In order to solve the problem of the occurrence of the roller mark, there is proposed an image forming apparatus which includes an air blowing hole between a fixing device and a conveying roller and solidifies a thermally fixed wax by blowing cooling air from the air blowing hole (see Japanese Patent Laid-Open No. 2003-21978).
Also, there is proposed an image forming apparatus configured such that a member such as a conveying roller is not brought into contact with a toner image until a temperature of a toner image is lower than a melting point of a wax (see Japanese Patent Laid-Open No. 2006-3404).
However, in the case of the image forming apparatus described in Japanese Patent Laid-Open No. 2003-21978, if a perimeter of a discharge portion side of the fixing device is warmed by a movement of a sheet or the like, a temperature of a fixed toner image rises, and thus, it may be difficult to cool down the temperature of the toner image to a melting point of a wax or lower before contact with the conveying roller. In this case, it is necessary to provide a more powerful cooling device or to perform cooling by reducing a sheet conveying speed. Also, the image forming apparatus described in Japanese Patent Laid-Open No. 2006-3404 has a problem in that since it takes time to perform cooling if a temperature of a toner image becomes high, a time for preventing the contact of the member such as the conveying roller is extended, leading to degraded productivity.
Therefore, it is desirable to provide an image forming apparatus which can prevent the occurrence of gloss unevenness, which is caused by a parting agent contained in a toner, through a simple structure, without degrading productivity.